Start SQLite db module

I plan to save all messages in an SQLite db.
For retrieving information it's nicer than having it in a text file.
We will have more info in there and easier to parse it.

This will also be good for later MAM
(https://github.com/profanity-im/profanity/issues/660).

Regular text files will still be an option for users so that they can
easily grep them and do whatever they like.

Internally Profanity will only use the SQLite db.

Makefile.am

@@ -1,5 +1,7 @@
 core_sources = \
-	src/xmpp/contact.c src/xmpp/contact.h src/log.c src/common.c \
+	src/xmpp/contact.c src/xmpp/contact.h \
+	src/log.c src/common.c \
+	src/database.h src/database.c \
 	src/log.h src/profanity.c src/common.h \
 	src/profanity.h src/xmpp/chat_session.c \
 	src/xmpp/chat_session.h src/xmpp/muc.c src/xmpp/muc.h src/xmpp/jid.h src/xmpp/jid.c \